Activation of naive T-cells requires TCR occupancy plus CD28-mediated co-stimulatory signals delivered by ligation of B7-1 and B7-2 on antigen presenting cells (APCs). In contrast, B7 ligation of CTLA-4 expressed on activated T-cells delivers a potent negative regulatory signal and may be involved in tolerance induction. These studies during the previous application period have clearly shown that strategies targeting these interactions have potent positive and negative regulatory activity on the initiation and progression of both acute and relapsing-remitting forms of experimental autoimmune encephalomyelitis (EAE), a CD4+ T-cell-mediated autoimmune disease which serves as a model for multiple sclerosis. The hypothesis under test in this renewal application is that the B7-CD28 co-stimulatory interactions play a critical role in positively regulating the activation and effector functions of autoreactive T-cells, while B7-CTLA-4 co-stimulatory interactions negatively regulate autoimmune disease and are required for the induction/maintenance of immune tolerance. Building on their preliminary data describing a new EAE model in NOD mice induced by priming with proteolipid protein peptide, PLP56-70, Aim 1 will employ a multifaceted approach, utilizing NOD mice with targeted deletion of B7-1, B7-2, B7-1/B7-2, or CD28 in comparison with antibody blocking studies, to further delineate the roles of the individual co-stimulatory receptors and ligands in the induction and effector phases of EAE and in activating myelin peptide-specific T-cell responses. This aim will also examine the individual roles of B7-1 and B7-2 in induction of cell surface activation and homing antigens, proliferation, patterns of cytokine/chemokine mRNA and protein expression, and the ability to activate wildtype T-cells and Th1 clones for adoptive transfer of R-EAE. In addition, they will explore the mechanistic basis behind the resistance of CD28 knockout mice to EAE induction. Aim 2 will expand their published studies examining the effects of modifying co-stimulatory signals in animals with a pre-existing autoimmune disease employing the PLP139-151-induced R-EAE model in SJL and (SJL x B10.PL)F1 mice in which relapses are due to the activation of encephalitogenic T-cells specific for endogenous; myelin epitopes induced by epitope spreading. Treatment of mice with intact anti-B7-1 mAb following the initial clinical episode results in a significant increase in relapse incidence and exacerbation of disease severity. However, similar treatment with the F(ab) fragments of anti-B7-1 mAb blocked epitope spreading, and significantly ameliorated CNS histopathology and prevented clinical relapses. The cellular targets and molecular mechanisms of intact anti-B7-1-mediated R-EAE exacerbation and anti-B7-1 F(ab) fragment-induced protection from disease relapses will be assessed by examining the Th1/Th2 phenotype and functional responses of peripheral and CNS-resident APCs and of T-cells specific for both the initiating and relapse-associated myelin epitopes. Aim 3 will explore the role of CTLA-4 in negatively regulating R-EAE pathogenesis and epitope spreading, and determine the role of B7-1, B7-2 and CTLA-4 in the induction and maintenance of peripheral tolerance induced by the i.v. injection peptide-pulsed, ECDI-fixed APCs. These studies should enhance our understanding of the role of co-stimulatory molecules in disease initiation and regulation of epitope spreading in chronic autoimmunity and provide vital information relative to the potential targeting of co-stimulatory molecules for treatment of pre-existing immune-mediated disorders.